Plyometric bench press is a good exchange exercise for the regular smith machine press when perhaps you havent quite got the 'go' to be doing heavy reps.  However, they're still challenging as they work a different sent of muscle fibres and also gives a cardio workout at the same time.  If you're a novice weight trainer, best to ask your instructor to help you as the exercise does need a certain degree of co-ordination.

Check out the video below on how to perform it. 

Make use of your surroundings!  I didn't want to stick one end of the barbell into the wall incase I leave a mark. So instead I used the T bar row to do the alternate barbell press.  Usually I'd do this standing up but bringing down to doing on my knees works the core more and makes the exerise even more harder.  Plus you get a great shoulder workout!

Check out the video below on how to perform it.